Greases as a kind of lubricant compositions are being extensively used for rolling members and sliding members of various industrial machines, vehicles, etc. However, especially in the aforementioned apparatus which are used under a high load or have rolling/sliding parts required to be lubricated, if the use conditions therefor become severe (load increase, oil film deficiency due to sliding friction, etc.), the lubrication of the rolling parts thereof, in particular, the rolling/sliding parts thereof, is apt to become boundary lubrication. As a result, galling, seizure due to thermal deterioration of the lubricant, etc. occur to significantly reduce the lubricity life of the parts. For maintaining satisfactory lubrication in such environments, it is indispensable to improve lubricity life by improving load bearing properties or reducing frictional resistance to inhibit heat generation, but the attainment thereof largely depends on the properties of greases.
For example, in tapered roller bearings, the lubricity life of the inner-ring cone back face rib and of the edge of each roller, which bear an axial load, becomes a problem. Namely, since the life of a tapered roller bearing is considerably influenced by the sliding speed and the contact area pressure at the rib, the grease used therein is required to inhibit heat generation and have load bearing properties. In a CVJ, a rolling/sliding movement occurs because the driving shaft on the differential gear side forms an angle with the idler shaft on the wheel side. For the reason, the lubrication in a CVJ is apt to become boundary lubrication and the friction generated therein influences the efficiency of power transmission and heat generation. Hence, use of a grease effective in improving frictional properties and inhibiting heat generation improves the performance of a CVJ and leads to the prolongation of durability life.
A generally employed expedient for mitigating the problem described above is to incorporate an extreme-pressure additive to a grease. Known extreme-pressure additives for greases include solid lubricants such as MoS.sub.2, sulfur, phosphorus, or sulfur-phosphorus organic compounds, organomolybdenum compounds such as a molybdenum dialkyldithiocarbamate (MoDTC) and a molybdenum dialkyldithiophosphate (MoDTP), and a zinc dialkyldithiophosphate (ZnDTP). MoDTC's, MoDTP's, and ZnDTP's are regarded as more effective than MOS.sub.2 and sulfur-phosphorus organic compounds.
Many proposals have been disclosed on the technique of incorporating extreme-pressure additives comprising organomolybdenum compounds or ZnDTP's into greases to obtain grease compositions having different properties according to the intended uses. For example, Examined Japanese Patent Publication No. 5-79280 discloses a technique of adding an MoDTC and an MoDTP to a urea grease to give a grease composition having reduced coefficient of friction, which is especially effective for improving the properties of plunging type CVJ's. Examined Japanese Patent Publication Nos. 4-34590 and 3-68920 and Unexamined Published Japanese Patent Application No. 60-47099 contain a description to the effect that extreme-pressure agents comprising an organomolybdenum or organozinc compound such as an MoDTC, MoDTP, or ZnDTP are especially effective.
It is further known that organoantimony compounds and ashless dialkylcarbamic acids are also effective.
On the other hand, greases comprising a mineral oil as the base oil have a drawback that they are more susceptible to oxidation than greases comprising as the base oil a synthetic lubricating oil, e.g., an ester oil, silicone oil, or ether oil, and in particular have a short lubricity life at high temperatures. Further, greases containing a lithium soap as a thickener, which are widely used as general-purpose greases, come to have a reduced oil-retaining ability at 130.degree. C. or higher because the grease structure thereof is destroyed at such high temperatures. Long-term use of these soap-containing greases results in a considerable decrease in lubricating action because the oxidation of the base oil is accelerated mainly by the catalytic function of the metal element contained in the soap.
In addition, once a grease is applied to various machines and apparatus, it is mostly used over a long period while being always in contact with air. Therefore, such a grease is desired to have better thermal and oxidative stability. From this point of view, as for bearings to be used at high temperatures and high speeds, a grease comprising a combination of a synthetic lubricating oil such as the aforementioned ones and a urea compound, which has excellent thermal and oxidative stability, is generally used.
As described above, maintenance cost has recently come to be taken in account, and with the recent trend toward size reduction and performance enhancement in apparatus, the use conditions therefor tend to become severer. With these trends, lubricant compositions are required to have a higher lubricating performance and a longer lubricity life.
However, the lubricant compositions containing an organomolybdenum compound, organozinc compound, organoantimony compound, or the like, which have hitherto been regarded preferable, cannot fully satisfy such a desire. In addition, the range of parts to which those prior art lubricant compositions are not applicable is increasing.
The present invention has been achieved in view of the circumstances described above. An object of the present invention is to provide lubricant compositions which have better load bearing properties and extreme-pressure properties than conventional ones, show an excellent lubricating performance at high temperatures, and prolong the lubricity life of parts lubricated therewith.